This invention relates to an electromagnetic selection device and, more particularly, to such a device for use with the needles of a knitting machine.
An electromagnetic selection device has been proposed, as suggested in U.S. Pat. No. 3,863,465, with a narrowing entry, a pair of opposing electromagnets, and a widening outlet. Selectable means, commonly referred to as selectors, magnetically adhere against the sides of such outlet. The magnetic adherence of the selectors is achieved by parts made of soft iron, associated with two permanent ferrite magnets located on both sides of a so-called selection canal, henceforth merely called the canal.
Although the proposed device appears to be satisfactory from a theoretical point of view, it is probable that, while in use, it might result in selection errors. The sizing of the ferrites, limited by problems of blocking up the selective device, does not guarantee the magnetic saturation of all selectors. Thus, when the same selectors, following at a short distance, are very unevenly distributed between the two sides of the canal, the magnetic field can be thrown out of balance and the adherence of the selector, then arriving on the proper side of the canal, cannot be ensured.
Another device of this kind, having a widening outlet, has its sides partially formed by two pole parts of opposing polarities and linked to a single permanent magnet. Pole parts of the same polarity are linked to one another by a magnetically permeable element, and pole parts of opposing polarities are separated by non-magnetic hard metal parts, resulting in differential magnetic adherence. Although this device is satisfactory, it presents substantial manufacturing drawbacks. For example, locating the exciting coil of the selection poles on a single core branch, besides being very complicated and having several gaps, creates a complex magnetic circuit having flux leakages which result in an assymetric selection flux for the selectors. Furthermore, the mechanical and magnetic tolerances of this device require the manufacturer to provide exact castings of each part in order to guarantee good performance of the device, even at high service temperatures due to the thermal dissipation of the electromagnet.